Spring Boot: POST request to entity with ManyToMany relationship

I'm working on a database for adding bands, musicians, instruments, etc.

I have a table 'band' and a table 'musician'. They have a ManyToMany relationship (one band can have many musicians, a musician can be in many bands), with an extra table BandMusician that has an embeddedId BandMusicianId. I did it like this because I want the relationship between bands and musicians to have also other information, like the year the musician joined the band.

@Entity
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Band {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    private String genre;
    private int year;

    @OneToOne(mappedBy = "band")
    private Website website;

    @OneToMany(mappedBy = "band")
    private List<Album> albuns;

    @OneToMany(mappedBy = "band")
    private List<BandMusician> musicians;

}


@Entity
@Data
@AllArgsConstructor
@NoArgsConstructor
@JsonDeserialize(using = MusicianJsonDeserializer.class)
public class Musician {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    @JsonFormat(pattern = "dd-MM-yyyy")
    @JsonProperty("DoB")
    @Column(name = "date_of_birth")
    private LocalDate DoB;

    @ManyToMany
    @JoinTable(
            name = "musician_instruments",
            joinColumns = @JoinColumn(name = "musician_id"),
            inverseJoinColumns = @JoinColumn(name = "instrument_id")
    )   
    private List<Instrument> instruments = new ArrayList<>();

    @OneToMany(mappedBy = "musician")
    private List<BandMusician> bands;

    public void addInstrument(Instrument instrument) {
        this.instruments.add(instrument);
    }

}


@Embeddable
@Data
@AllArgsConstructor
@NoArgsConstructor
public class BandMusiciansId implements Serializable{
    
    private static final long serialVersionUID = 1L;

    @Column(name = "band_id")
    private Long bandId;

    @Column(name = "musician_id")
    private Long musicianId;
}


@Entity
@Data
@AllArgsConstructor
@NoArgsConstructor
public class BandMusician {

    @EmbeddedId
    private BandMusiciansId id = new BandMusiciansId();

    @ManyToOne
    @MapsId("bandId")
    @JoinColumn(name = "band_id")
    private Band band;

    @ManyToOne
    @MapsId("musicianId")
    @JoinColumn(name = "musician_id")
    private Musician musician;

    private String role;
    private int joined;
}

When I receive a POST request to "/musician" I can save a musician. I'm using Jackson to deserialize a request like this:

{
    "name": "John the Ripper",
    "DoB": "03-12-1965",
    "instruments": "voice, guitar",
    "bands": "Band1, Band2"
}

With Jackson I can get each band, search with the BandRepository and create a BandMusician.

THE PROBLEM: When I receive the request, in order to create a BandMusician I have to create a BandMusiciansId, and to do that I need the bandId and the MusicianId. But I'm creating the musician right now, so I don't have the musicianId. It is created automatically when I save the musician.

MusicianJsonDeserializer class

public class MusicianJsonDeserializer extends JsonDeserializer<Musician>{

private final InstrumentRepository instrumentRepository;
private final BandRepository bandRepository;

@Autowired
public MusicianJsonDeserializer(
        InstrumentRepository instrumentRepository,
        BandRepository bandRepository
) {
    this.instrumentRepository = instrumentRepository;
    this.bandRepository = bandRepository;
}

@Override
public Musician deserialize(JsonParser p, DeserializationContext ctxt) 
        throws IOException, JacksonException {
    
    ObjectCodec codec = p.getCodec();
    JsonNode root = codec.readTree(p);
    
    Musician musician = new Musician();
    musician.setName(root.get("name").asText());
    
    DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd-MM-yyyy");
    musician.setDoB(LocalDate.parse(root.get("DoB").asText(), formatter));
    
    if (root.get("instruments") != null) {
        String instrumentList = root.get("instruments").asText();
        String[] instrumentArray = instrumentList.split(", ");
        List<Instrument> musicianInstrumentList = new ArrayList<>();
        
        for (String instrument : instrumentArray) {
            Instrument instrumentFound = 
                    instrumentRepository.findByName(instrument)
                    .orElseThrow(RuntimeException::new);
            // TODO custom exception
            musicianInstrumentList.add(instrumentFound);
        }
        
        musician.setInstruments(musicianInstrumentList);
    }
    
    if (root.get("bands") != null) {
        // TODO Stuck here!

What I thought of doing: In my MusicianService, after saving the musician, I can create the BandMusician and the relationship. I think doing this in the Service layer would be a bad choice though.

EDIT: To make it easier to understand, I created a project only with the relevant parts of this one and pushed to github (https://github.com/ricardorosa-dev/gettinghelp). Again, what I want is to be able to send a POST to "/musician", that will be caught by the MusicianJsonDeserializer, and somehow create a BandMusicianId and BandMusician for each band sent in the request body.